Common devices for propelling watercraft include submerged rotating propellers, waterjets and the like. A conventional rotating propeller rarely presents an efficiency above 20%, and will also suffer from cavitation at higher rotary speeds. The very low efficiency of a conventional rotating propeller results in high operation costs and a strong impact on the environment from the emissions of any combustion engine powering the propeller. A conventional rotating propeller is also prone to get entangled with fishing net, rope, line, plastic sheet or other floating debris, and may cause harm to swimmers and animal life that come near the propeller. In the disclosure below, the term rotatable propeller refers to the above mentioned conventional, helical or screw type rotatable propellers that are predominant today and known well to those skilled in the art.
An example of a propulsion device comprising a rotatable propeller is disclosed in the European patent publication EP1852589A2.